Birefringent optical component

ABSTRACT

An optical component ( 600, 181 ) comprises two adjacent materials ( 610, 620 ) with a shaped (e.g. curved) interface between the materials ( 612, 622 ). The first of the materials ( 610 ) is birefringent. The second material ( 620 ) has a refractive index substantially equal to the refractive index of the birefringent material at a predetermined angle.

FIELD OF THE INVENTION

The present invention relates to optical components comprising abirefringent material, devices including such components and methods ofmanufacturing such components and devices. The component is particularlysuitable for but not limited to, use as an optical element in opticalscanning devices.

BACKGROUND OF THE INVENTION

Optical pickup units for use in optical scanning devices are known. Theoptical pickup units are mounted on a movable support for scanningacross the tracks of the optical disk. The size and complexity of theoptical pickup unit is preferably reduced as much as practicable, inorder to reduce the manufacturing cost and to allow additional space forother components being mounted in the scanning device.

Modern optical pickup units are generally compatible with at least twodifferent formats of optical disk, such as the Compact Disc (CD) and theDigital Versatile Disc (DVD) format. Recently proposed has been theBlu-ray Disk (BD) format, offering a data storage capacity of around 25GB (compared with a 650 MB capacity of a CD, and a 4.7 GB capacity of aDVD).

Larger capacity storage is enabled by using small scanning wavelengthsand large numerical apertures (NA), to provide small focal spots, (thesize of the focal spot is approximately λ/NA), so as to allow thereadout of smaller sized marks in the information layer of the disk. Forinstance, a typical CD format utilises a wavelength of 785 nm and has anobjective lens with a numerical aperture of 0.45, a DVD uses awavelength of 650 nm and has a numerical aperture of 0.65, and a BDsystem uses a wavelength of 405 nm and a numerical aperture of 0.85.

Typically, the refractive index of materials vary as a function ofwavelength. Consequently, a lens will provide different focal points anddifferent performance for different incident wavelengths. Further, thediscs may have different thickness transparent layers, thus requiring adifferent focal point for different types of discs.

In some instances, storage capacity is further increased by increasingthe number of information layers per disc. For example, a dual layerBD-disc has two information layers separated by a 25 μm thick spacerlayer. Thus, the light from the optical pickup unit has to travelthrough the spacer layer when focusing on the second information layer.This introduces spherical aberration, the phenomenon that rays close tothe axis of the converging cone of light have a different focal pointcompared to the rays on the outside of the cone. This results in ablurring of the focal spot, and a subsequent loss of fidelity in theread-out of the disc.

To enable dual layer readout and backward compatibility (i.e. the sameoptical system being used for different disc formats), polarisationsensitive lenses (PS-Lenses) have been proposed to compensate forspherical aberration. Such lenses can be formed of a birefringentmaterial, such as a liquid crystal. Birefringence denotes the presenceof different refractive indices for the two polarisation components of abeam of light. Birefringent materials have an extraordinary refractiveindex (n_(e)) and an ordinary refractive index (n_(o)), with thedifference between the refractive indices being Δn≈n_(e) 31 n_(o). PSlenses can be used to provide different focal points for a single ordifferent wavelengths by ensuring that the same or differentwavelength(s) are incident upon the lens with different polarisations.

It is an aim of embodiments of the present invention to provide animproved optical component which addresses one or more of the problemsof the prior art, whether referred to herein or otherwise.

It is an aim of particular embodiments of the present invention toprovide a birefringent lens that can be switched to a neutral state suchthat it does not alter the direction of incident light, as well as amethod of manufacturing such a lens.

STATEMENTS OF THE INVENTION

In a first aspect, the present invention provides an optical scanningdevice for scanning an information layer of an optical record carrier,the device comprising a radiation source for generating a radiationbeam, and an objective system for converging the radiation beam on theinformation layer, wherein the device includes an optical elementcomprising at least two adjacent materials with a shaped interfacebetween the materials, at least the first of the materials beingbirefringent, the second material having a refractive indexsubstantially equal to the refractive index of the birefringent materialat a predetermined angle.

By providing an element having two such materials, the optical functiondefined by the interface can effectively be switched to a neutral state.For instance, if the interface is curved, the lens capability of theinterface can be switched so as not to provide any focussing ordispersing effect by ensuring that a polarised beam of radiation isincident upon the element with the correct orientation. This permits thesimplification of the optical arrangement within a scanning device.Further, the second material can act to protect, at least in part, thebirefringent material.

In another aspect, the present invention provides an optical componentcomprising at least two adjacent materials with a shaped interfacebetween the materials, at least the first of the materials beingbirefringent, the second material having a refractive indexsubstantially equal to the refractive index of the birefringent materialat a predetermined angle.

In a further aspect, the present invention provides a method ofmanufacturing an optical scanning device for scanning an informationlayer of an optical record carrier, the information layer being coveredby a transparent layer of thickness t_(d) and refractive index n_(d),the method comprising the steps of: providing a radiation source forgenerating a radiation beam; providing an optical element, the opticalelement comprising at least two adjacent materials with a shapedinterface between the materials, at least the first of the materialsbeing birefringent, the second material having a refractive indexsubstantially equal to the refractive index of the birefringent materialat a predetermined angle.

In another aspect, the present invention provides a method ofmanufacturing an optical component, the method comprising: providing atleast two adjacent materials with a shaped interface between thematerials, at least the first material being birefringent and the secondmaterial having a refractive index substantially equal to one of therefractive indices of the birefringent material at a predeterminedangle.

BRIEF DESCRIPTION OF DRAWINGS

For a better understanding of the invention, and to show how embodimentsof the same may be carried into effect, reference will now be made, byway of example, to the accompanying diagrammatic drawings in which:

FIG. 1 illustrates an optical component in accordance with a preferredembodiment of the present invention;

FIGS. 2A-2H illustrate method steps in the formation of a liquid crystallens in accordance with a preferred embodiment of the present invention;

FIG. 3 illustrates a device for scanning an optical record carrierincluding a liquid crystal lens in accordance with an embodiment of thepresent invention;

FIGS. 4A and 4B illustrate how the optical system of the scanning deviceshown in FIG. 3 may be used with different polarisations of light toscan different layers within a dual layer optical record carrier, and

FIG. 5 illustrates an optical component in accordance with a further 5embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Optical components (or portions of optical components, optical elements)can include curved surfaces so as to focus light (e.g. a convex lens) ordisperse light (e.g. a concave lens). Birefringent optical componentswith curved surfaces will provide different focussing or dispersiveeffects, dependent upon the angle at which the polarised radiation beamis incident on the optical component.

Equally, optical functions of other components are provided by othershaped (i.e. non-planar) surfaces such as step functions and gratings.

The present inventor has realised that, by providing an additionalmaterial adjacent to the curved (or otherwise shaped) surface, with theadditional material having a refractive index substantially equal to therefractive index of the birefringent material at a predetermined angle,then when polarised light is incident on the surface (i.e. the interfacebetween the birefringent material and the additional material) at thispredetermined angle, the surface will have a neutral effect (e.g. itwill not act to focus or disperse the light) due to the index matching.

Consequently, for differently shaped surfaces, such as step structuresand gratings, the optical function of the components can be switched onand off by setting the incident polarisation such that it leads to asubstantially equal refractive index match between the two adjacentmaterials, so that the interface between the two materials becomesinvisible.

In inorganic birefringent materials (e.g. a crystal such as calcite) theatomic structure is non-symmetric. This leads to an anisotropy in thephysical constants of materials in different directions. One of those isthe refractive index. Consider a polarised beam of light traversingalong different optical axis. There will be one optical axis in which adifferent refractive index will be observed upon traversionperpendicularly and parallel to the optical axis. In general, but notalways, two out of three axes have a refractive index that is higherthan the refractive index of the third axis.

In organic crystals, such as a liquid crystal, a similar phenomenonoccurs although one can of course not talk about a difference in theatomic structure but only of orientational order within the liquid thatresembles a crystal structure. Generally, although not always, two outof three axes have a refractive index that is lower than in the thirdaxis.

The direction in which the molecules of a liquid crystal are aligned iscalled the director. Light propagating with its plane of polarisationparallel to the director experiences the extraordinary refractive index,n_(e).

FIG. 1 illustrates an optical component 600 in accordance with apreferred embodiment of the present invention. The optical component 600can be envisaged as being formed of two portions. The first portion is aplanoconvex lens 610 formed of birefringent material. Since thebirefringent material is made of a typical liquid crystal, it has twoordinary axes yielding an ordinary refractive index n_(o) and oneextraordinary axis yielding a refractive index n_(e). The second portionof the component comprises a planoconcave lens 620. In this embodiment,the planoconcave lens is formed of a material having a uniformrefractive index n_(s), where n_(e)≧n_(s)≧n_(o). In this particularembodiment, n_(s)=n_(o). The extraordinary axis of the birefringentmaterial is perpendicular to the normal of the component.

The curved interface between the two portions corresponds to the convexsurface 612 of the planoconvex lens 610 mating with the concave surface622 of the concave lens 620.

It will be appreciated that, when the polarised light is incident uponthe optical component 600 along the ordinary axis of the birefringentmaterial with its plane of polarisation perpendicular to the director,then as n_(o)=n_(s), the light will not experience any lens effects i.e.the component will act as an optically neutral component.

However, when the plane of polarisation of the polarised light incidentupon the optical component 600 is no longer perpendicular to thedirector, the refractive index of the planoconvex 610 portion will begreater than the refractive index of the planoconcave portion 620. Thisis valid only for the plane of polarisation projected onto theextraordinary axis of the birefringent material, such that for thisprojected polarisation a lens effect is realised by the light i.e. thelight is focused. For the plane of polarisation projected onto theordinary axis of the birefringent material no refractive indextransition is observed.

Since the plane of polarisation is projected onto two axes, twoindividual lens effects will be realised, which if desired can be madevisible separately using a polariser.

When the plane of polarisation is exactly parallel to the director andthe angle of incidence is exactly parallel to the normal of the opticalcomponent, there is no projection of the plane of polarisation onto theordinary axis and thus only the n_(e) is experienced for thebirefringent material. Maximum light intensity is then achieved in onesingle spot, so the light is focused.

In another case where the component is tilted at an angle θ with respectto the normal of the component, but without twist, such that the planeof polarisation intersects with the extraordinary axis of thebirefringent material, a refractive index n_(θ) is observed according tothe formula:$n_{\theta} = \frac{n_{o}n_{e}}{\sqrt{{n_{e}^{2}\sin^{2}\theta} + {n_{o}^{2}\cos^{2}\theta}}}$

FIGS. 2A-2H illustrate respective steps in forming an optical componentin accordance with a preferred embodiment of the present invention. Inthis particular instance, the optical component includes a liquidcrystal birefringent lens.

In the first step, shown in FIG. 2A, mould 100 is provided, the mouldhaving a shaped surface 102 which subsequently serves to define aportion of the shape of the resulting optical component. In thisparticular instance, the liquid crystal is ultimately photopolymerised,and consequently the mould is formed of a material transparent to theradiation used to polymerise the liquid crystal e.g. glass.

An alignment layer 110 is arranged on the curved surface 102, so as toinduce a predetermined orientation (indicated by the arrow direction110) in the liquid crystal subsequently placed upon the alignment layer.

In this particular example, the alignment layer is a layer of polyimide(PI). The polyimide may be applied using spincoating from a solution.The polyimide may then be aligned so as to induce a specific orientation(this orientation determining the resulting orientation of the liquidcrystal molecules). For instance, a known process is to rub thepolyimide layer with a non-fluff cloth repeatedly in a single directionso as to induce this orientation (110).

A substrate 150, which in this particular embodiment will form part ofthe optical component, has a bonding layer 120 applied to a firstsurface 152. The bonding layer is arranged to form a bond with theliquid crystal. In this particular instance, the bonding layer is alsoan alignment (or orientation) layer comprising polyimide. The bondinglayer contains reactive groups arranged to form a chemical bond with theliquid crystal molecules, and in this instance has the same type ofreactive group as the liquid crystal molecules, such that whenphotopolymerising the liquid crystal molecules, chemical bonds with thebonding layer on the substrate are also created. This results in verygood adhesion between substrate and the liquid crystal layer. Thebonding layer may be deposited on the substrate using the same type ofprocess used to deposit and align the alignment layer on the mould 100.The bonding layer, which in this instance also functions as an alignmentlayer, is oriented in a predetermined orientation (arrow 120) dependingupon the desired properties of the resulting liquid crystal components.

The bonding layer is aligned so as to be parallel to the direction 110of the alignment layer on the mould. Preferably, the orientation of thebonding layer is parallel but in the opposite direction to theorientation of the alignment layer.

As illustrated in FIG. 2B, a compound 200 incorporating one or moreliquid crystals is then placed between the first surface 152 of thesubstrate 150 and the shaped surface 102 of the mould 100.

In this particular example, as illustrated in FIG. 2B, the compound 200comprises a mixture of two different liquid crystals. These twodifferent liquid crystals have been chosen so as to provide the desiredrefractive index properties once at least one of the liquid crystals hasbeen polymerised.

A droplet of the liquid crystals 200 is placed on the first surface 152of the substrate. The compound 200 has been degassed, so as to avoid theinclusion of air bubbles within the resulting optical component. It alsoavoids the formation of air bubbles from dissolved gases coming out ofthe solidifying liquid during polymerisation, as the shrinkage duringpolymerisation leads to a large pressure decrease inside thepolymerising liquid.

The glass mould is then heated so that the liquid crystal is in theisotropic phase (typically to about 80° C.), so as to facilitate thesubsequent flow of the liquid crystal into the desired shape.

The substrate and the mould are subsequently brought together, so as todefine the shape of the liquid crystal portion 201 of the finalresulting optical component (FIG. 2C). In order to ensure that theliquid crystal forms a homogenous layer between the mould and thesubstrate, a pressure may be applied to push the substrate towards themould (or vice versa).

The substrate/mould/liquid crystal may then be cooled, for instance downto room temperature for 30 minutes, so as to ensure that the liquidcrystal enters the nematic phase, coming from the isotropic phase.

When entering the nematic stage, multi domains may appear in the liquidcrystal mixture. Consequently, the mixture can be heated to above theclearing point to destroy the multidomain orientation (e.g. the mixturemay be heated for 3 minutes to 105° C.). Subsequently, the mixture maybe cooled to obtain a homogenous orientation 202 (FIG. 2D).

The homogenous liquid crystal mixture may then be photopolymerised usinglight 302 from an ultra violet radiation source 300 (FIG. 2E), forinstance by applying a UV-light intensity of 10 W/cm² for 60 seconds. Atthe same time, chemical bonds will be formed between the liquid crystaland the bonding layer.

Subsequently, the first element (or portion) of the optical component(150, 203) can be released from the mould 100 (FIG. 2F). This could, forinstance, be achieved by slightly bending the mould 100 over a corneredobject 400. Alternatively, it could be achieved by pressing a portion ofthe flat substrate in a flat support, so as to slightly bend the flatsubstrate. The liquid crystal/substrate element should separate easilyfrom the mould, as a conventional polyimide (without reactive groups) isused on the mould.

The mould can be reused to produce subsequent elements of components, byrepeating steps illustrated in FIGS. 2B-2F. Typically, the alignmentlayer will remain upon the mould 100, and hence does not need to bereapplied.

If desired, a further processing step can be performed to remove theliquid crystal 202 from the substrate 150. However, in most instances itis assumed that the substrate 150 will form part of the final opticalcomponent.

FIGS. 2G and 2H illustrate the processing steps that can be used toprovide the second material to the optical element formed by steps2A-2F, so as to result in the final optical component.

A second substrate 160 is provided with a liquid substance that can beturned into a transparent solid with the desired refractive index e.g. acurable monomer 162. Spacers 170 are placed on top of the firstsubstrate 150 (i.e. on the same side of the substrate as the polymerisedbirefringent element 203). The spacer act to define the gap between thesurface of the liquid crystal and the flat surface of the polymerisedmonomer layer. These spacers could also act to define the length of thefinal optical component. In this particular example, the final opticalcomponent has a length equal to the width of substrate 150, the width ofsubstrate 160, and the height of the spacers 170.

The curable monomer 162 has been selected such that the refractive indexof the monomer after curing will be substantially equal to the ordinaryrefractive index of the polymerised birefringent material 203.

The second substrate 160 can be formed of a transparent material, suchas glass. The spacers can be formed of any desired material, forinstance glass or foil.

As shown in FIG. 2H, the second substrate 160 is placed upon the spacers170, so as to sandwich the curable monomer 162 from FIG. 2G between thetwo substrates 150, 160. The monomer will then fill the gap between thetwo substrates.

Subsequently, the monomer 162 is cured to form the polymer 164 byapplying UV radiation 302 from a UV radiation source 300.

Subsequently, if desired, either or both of the substrates 150, 160 maybe removed.

The result is an optical component, generally similar to thatillustrated in FIG. 1.

A suitable polyimide for use in the alignment layer is OPTMER AL-1051supplied by Japan Synthetic Rubber Co., whilst Merck ZLI2650, spincoatedfrom a solution in γ-butyrolactone can be used as an appropriatereactive polyimide with methacrylate groups as the bonding layer.

As mentioned above, in the preferred embodiment a mixture of two liquidcrystals was utilised to obtain the desired n_(e) and n_(o). The twoliquid crystals utilised were1,4-di(4-(3-acryloyloxypropyloxy)benzoyloxy)-2-methylbenzene (RM 257)and E7 (a cyanobiphenyl mixture with a small portion of cyanotriphenylcompound) both from Merck, Darmstadt, Germany. The photoinitiator usedto ensure the photo polymerisation of both the liquid crystals and thecurable monomer was Irgacure 651, obtainable from Ciba Geigy, Basel,Switzerland The curable monomer used was2,2-di(4-(2-methacryloyloxyethyloxy)phenoxy)-propane (Diacryl 101) fromAkzo Nobel, Arnhem, The Netherlands.

In some instances, a surfactant was mixed with the liquid crystal topromote the lens release from the mould. The surfactants utilised wereFC171 a perfluorinated surfactant (3M) and 2-(N-ethylperfluorooctanesulfonamido-ethylacrylate (Acros). The use of the surfactant was seen toinfluence the orientation of the liquid crystal (a lower Δn was seenwhen a surfactant was utilised).

FIG. 3 shows a device 1 for scanning an optical record carrier 2,including an objective lens 18 according to an embodiment of the presentinvention. The record carrier comprises a transparent layer 3, on oneside of which an information layer 4 is arranged. The side of theinformation layer facing away from the transparent layer is protectedfrom environmental influences by a protection layer 5. The side of thetransparent layer facing the device is called the entrance face 6. Thetransparent layer 3 acts as a substrate for the record carrier byproviding mechanical support for the information layer.

Alternatively, the transparent layer may have the sole function ofprotecting the information layer, while the mechanical support isprovided by a layer on the other side of the information layer, forinstance by the protection layer 5 or by a further information layer anda transparent layer connected to the information layer 4. Informationmay be stored in the information layer 4 of the record carrier in theform of optically detectable marks arranged in substantially parallel,concentric or spiral tracks, not indicated in the Figure. The marks maybe in any optically readable form, e.g. in the form of pits, or areaswith a reflection coefficient or a direction of magnetisation differentfrom their surroundings, or a combination of these forms.

The scanning device 1 comprises a radiation source 11 that can emit aradiation beam 12. The radiation source may be a semiconductor laser. Abeam splitter 13 reflects the diverging radiation beam 12 towards acollimator lens 14, which converts the diverging beam 12 into acollimated beam 15. The collimated beam 15 is incident on an objectivesystem 18.

The objective system may comprise one or more lenses and/or a grating.The objective system 18 has an optical axis 19. The objective system 18changes the beam 17 to a converging beam 20, incident on the entranceface 6 of the record carrier 2. The objective system has a sphericalaberration correction adapted for passage of the radiation beam throughthe thickness of the transparent layer 3. The converging beam 20 forms aspot 21 on the information layer 4. Radiation reflected by theinformation layer 4 forms a diverging beam 22, transformed into asubstantially collimated beam 23 by the objective system 18 andsubsequently into a converging beam 24 by the collimator lens 14. Thebeam splitter 13 separates the forward and reflected beams bytransmitting at least part of the converging beam 24 towards a detectionsystem 25. The detection system captures the radiation and converts itinto electrical output signals 26. A signal processor 27 converts theseoutput signals to various other signals.

One of the signals is an information signal 28, the value of whichrepresents information read from the information layer 4. Theinformation signal is processed by an information processing unit forerror correction 29. Other signals from the signal processor 27 are thefocus error signal and radial error signal 30. The focus error signalrepresents the axial difference in height between the spot 21 and theinformation layer 4. The radial error signal represents the distance inthe plane of the information layer 4 between the spot 21 and the centreof a track in the information layer to be followed by the spot.

The focus error signal and the radial error signal are fed into a servocircuit 31, which converts these signals to servo control signals 32 forcontrolling a focus actuator and a radial actuator respectively. Theactuators are not shown in the Figure. The focus actuator controls theposition of the objective system 18 in the focus direction 33, therebycontrolling the actual position of the spot 21 such that it coincidessubstantially with the plane of the information layer 4. The radialactuator controls the position of the objective lens 18 in a radialdirection 34, thereby controlling the radial position of the spot 21such that it coincides substantially with the central line of track tobe followed in the information layer 4. The tracks in the Figure run ina direction perpendicular to the plane of the Figure.

The device of FIG. 3 in this particular embodiment is adapted to scanalso a second type of record carrier having a thicker transparent layerthan the record carrier 2. The device may use the radiation beam 12 or aradiation beam having a different wavelength for scanning the recordcarrier of the second type. The NA of this radiation beam may be adaptedto the type of record carrier. The spherical aberration compensation ofthe objective system must be adapted accordingly.

FIGS. 4A and 4B illustrate how the polarisation sensitive lensmanufactured in accordance with the above embodiment can be utilised toprovide two different focal points, suitable for reading a dual-layeroptical recording medium 2′. The dual-layer medium 2′ has twoinformation layers (4, 4′), a first information layer 4 at a depth dwithin the transparent layer 3, and a second information layer 4′ afurther distance Δd beneath the first information layer 4.

In the embodiment shown in FIGS. 4A and 4B, the objective system 18comprises a polarisation sensitive lens 181 (comprising liquid crystal203, and manufactured as described above), a second lens 182, aquarter-wave (λ/4) plate 183, and a twisted nematic TN) liquid crystalcell 184.

The focal point of the objective system can be altered by using thebifocal nature of the liquid crystal lens 181.

In the off mode, the TN-cell acts to rotate the polarisation of incidentradiation by 90°. For instance, as shown in FIG. 4A, when the TN-cell isoff, then incident p-polarised radiation will be rotated by 90° to forms-polarised radiation.

The twisted nematic cell thus acts as a beam rotation means arranged tocontrollably alter the angle at which the polarised radiation beam isincident on the optical element 181. As an alternative, it will beappreciated that the optical element 181 could instead be rotated, withthe polarised radiation beam remaining stationary.

It is assumed that, due to the particular orientation of thebirefringent material within the optical element 181, when thes-polarised radiation is incident on the element 181, the radiationexperiences the ordinary refractive index of the birefringent material.As, in this particular example, the ordinary refractive index is equalto the refractive index of the second portion of the optical element,the optical element 181 acts as an optically neutral element tos-polarised radiation. In other words, if the s-polarised radiation is aparallel beam incident upon the element 181, then it exits the elementas a parallel beam.

After the optical element 181, the s-polarised beam is incident upon thequarter-wave plate, which acts to change the s-polarised beam to righthand circularly polarised light (RHC), which is focused on to the secondinformation layer 4′. Upon reflection from the layer, the RHC light isconverted to left hand circularly polarised light (LHC). The LHC light,upon being transmitted through the quarter-wave plate, is converted top-polarised light. The p-polarised light then passes back through theoptical element 181, and is changed to s-polarised light by the TN-cell184.

As shown in FIG. 4A, this means that when p-polarised light enters theobjective system 18, the light is incident upon the information layer4′, and the reflective light leaves as s-polarised light from theobjective system 18. Alternatively when s-polarised light enters theobjective system 18, the light is incident upon the information layer 4and the reflected light leaves the objective system as p-polarised.Consequently, if the beam splitter 13 shown in FIG. 3 is a polarisingbeam splitter, it is easy to ensure that no reflected light is directedback towards the light source 11, but almost all reflected is directedtowards the detector 25 since most polarising beam splitters transmitp-polarised light and reflect s-polarised light.

In FIG. 4B, the same optical arrangement exists, but in this figure theTN-cell is on, e.g. by applying a sufficiently high voltage over thecell, such that the TN-cell does not change the polarisation of lightpassing through it. Consequently, p-polarised light is incident upon theoptical element 181. The p-polarised light thus experiences a change inrefractive index when passing from the second portion of the element 181to the first portion of the element i.e. it experiences some focusing(convergence) due to the planoconvex birefringent lens that forms thefirst portion of the element 181.

The p-polarised light, which is now slightly converging, is thenincident upon the quarter-wave plate 183. The quarter-wave plate acts tochange the p-polarised light to LHC light, which is further focused bythe lens 182 so as to be incident upon the first information layer 4.Upon reflection from the first information layer 4, the LHC light turnsinto RHC light. The RHC light, as it passes through the quarter-waveplate 183, is then changed to s-polarised light, which subsequentlypasses back through the optical element 181 and the TN-cell 184.

Thus, as shown in FIGS. 4A and 4B, an optical element may be provided inaccordance with an embodiment of the present invention in a scanningdevice. The element 181 may function as a neutral optical component (asshown in FIG. 4A), or as a focusing element (as shown in FIG. 4B). Suchan element, as it is optically neutral, emits relatively easy beamshaping within the scanning device.

It will be appreciated that the above embodiments are described by wayof example only, and that various alternatives will be apparent to theskilled person.

The mould used in the manufacturing process may be formed of anymaterial, including rigid materials such as glass.

Further, the shaped surface of the mould may be dimensioned so as toallow for any change in shape or volume of the liquid crystal materialduring the method. For instance, typically liquid crystal monomersshrink slightly upon polymerisation, due to double bonds within theliquid crystal being reformed as single bonds. By appropriately makingthe optical component shaped defined by the substrate and the mouldslightly oversize, an appropriately sized and shaped optical componentcan be produced.

Whilst the substrates have been seen in this particular example ascomprising a single sheet of glass, with two flat, substantiallyparallel sides, it will be appreciated that the substrates can in factbe any desired shape.

An extra adhesion layer may be applied to the mould and/or substrate(prior to deposition of the bonding layer onto the substrate and theorientation layer to the mould), so as to make sure that the appliedlayers are well attached to the mould and the substrate. For instance,organosilanes may be used to provide this adhesion layer. For thesubstrate an organosilane comprising a methacrylate group may be usedand for the mould an organosilane comprising an amine end group may beused.

It will be appreciated that the above described optical components aredescribed by way of example only. An optical component (or indeed, anoptical element formed according to the present invention i.e. a portionof an optical component) could be formed with different properties tothat described above.

For instance, in the above embodiments, it is assumed that therefractive index n_(s) of the second portion of the component 620 isequal to n_(o). However, it will be appreciated that in fact any valueof n_(s) could be used, provided either n_(e)≧n_(s)≧n_(o) orn_(e)≦n_(s)≦n_(o). For instance, an optical component could be formedwith n_(s)=n_(e).

Alternatively, n_(s) could be any fixed, predetermined value betweenn_(o) and n_(e). In such an instance, the optical element could beenvisaged as having three separate modes of operation, depending uponthe refractive index n_(θ) experienced by the polarised electromagneticradiation beam as it passes through the birefringent material at anangle θ. The three modes will thus correspond to (I) n_(θ)<n_(s), (II)when n_(θ)=n_(s), and (III) when n_(θ)>n_(s). In each instance, theeffect (power) of the curved interface within the optical element on theradiation will vary depending upon the differences between n_(s) andn_(θ).

Equally, whilst in the above embodiments the optical component has beendescribed as having a curved interface between the two materials, itwill be appreciated that the interface could in fact be of any shapethat provides an optical function. For instance, the interface could bea step structure or a grating structure. In such instances, the opticalfunctions of the components can still be switched on and off by settingthe incident polarisation such that it leads to a substantially equalrefractive index match between the two adjacent materials.

Whilst specific examples of materials suitable for forming the opticalcomponent have been described, and particular manufacturing steps, theseare again provided by way of example only.

Equally, in the above embodiment it has been assumed that the secondportion 620 of the optical element has a uniform refractive index n_(s),which is not polarisation dependent. However, it will be appreciatedthat in fact the second portion 620 could be formed of a birefringentmaterial, as long as the criteria is satisfied that at a particularangle of incidence, the refractive index of the second portion 620 isequal to the refractive index of the first portion 610.

In the preferred embodiment, it is assumed that the outer surfaces ofthe optical element (i.e. the surfaces upon which the light enters andexits the element) are two flat, parallel surfaces. However, thesesurfaces could in fact be any desired shape, including concave orconvex.

For instance, FIG. 5 illustrates an optical element 400 in accordancewith a further embodiment of the present invention. In this embodiment,the optical element comprises a first portion 402 formed of abirefringent material, and a second portion 404 formed of a materialhaving a refractive index equal to the extraordinary refractive index ofthe birefringent material. However, in this particular embodiment, thebirefringent material is formed as a convex lens, rather than aplanoconvex lens. As previously, the second portion of the opticalelement in this instance is formed as a planoconcave lens mated with onesurface of the convex lens portion.

In all of the above embodiments of the optical component, the shapedinterface between the two materials of the component can, for anappropriate angle of incidence polarised radiation, be opticallyneutral. This allows the optical element to be used in a number of noveland interesting ways.

1. An optical scanning device for scanning an information layer of anoptical record carrier, the device comprising a radiation source forgenerating a radiation beam and an objective system for converging theradiation beam on the information layer, wherein the device includes anoptical element comprising at least two adjacent materials with a shapedinterface between the materials, at least the first of the materialsbeing birefringent, the second material having a refractive indexsubstantially equal to the refractive index of the birefringent materialat a predetermined angle.
 2. A device as claimed in claim 1, wherein theradiation source is arranged to generate a polarised radiation beam, theoptical scanning device further comprising beam rotation means arrangedto controllably alter the angle at which the polarised radiation beam isincident on the optical element.
 3. A device as claimed in claim 2,wherein said beam rotation means is arranged to rotate the element.
 4. Adevice as claimed in claim 2, wherein said beam rotation means isarranged to alter the polarisation angle of the polarised radiationbeam.
 5. A device as claimed in claim 1, wherein said second material isbirefringent.
 6. A device as claimed in claim 1, wherein the secondmaterial has a refractive index n_(s) and the birefringent material hasan ordinary refractive index n_(o) and an extraordinary refractive indexn_(e), wherein n_(e)≧n_(s)≧n_(o) or n_(e)≦n_(s)≦n_(o).
 7. A device asclaimed in claim 1, wherein at least one of the first material and thesecond material is shaped as a lens.
 8. A device as claimed in claim 1,wherein at least of said first material and said second material isshaped as at least one of a planoconcave lens and a planoconvex lens. 9.A device as claimed in claim 1, wherein one of the two materials isshaped as a planoconvex lens and the other of the two materials isshaped as a mating planoconcave lens.
 10. An optical componentcomprising at least two adjacent materials with a curved interfacebetween the materials, at least the first of the materials beingbirefringent the second material having a refractive index substantiallyequal to the refractive index of the birefringent material at apredetermined angle.
 11. An optical element as claimed in claim 10,wherein said interface is curved.
 12. An optical component as claimed inclaim 10, wherein said first material comprises a polymerisedanisotropically oriented liquid crystal.
 13. An optical component asclaimed in claim 10, wherein at least one of the outer surfaces of theoptical element is planar.
 14. A method of manufacturing an opticalscanning device for scanning an information layer of an optical recordcarrier, the information layer being covered by a transparent layer ofthickness t_(d) and refractive index n_(d), the method comprising thesteps of: providing a radiation source for generating a radiation beam;providing an optical element, the optical element comprising at leasttwo adjacent materials with a shaped interface between the materials, atleast the first of the materials being birefringent, the second materialhaving a refractive index substantially equal to the refractive index ofthe birefringent material at a predetermined angle.
 15. A method ofmanufacturing an optical component, the method comprising: providing atleast two adjacent materials with a shaped interface between thematerials, at least the first material being birefringent and the secondmaterial having a refractive index substantially equal to one of therefractive indices of the birefringent material at a predeterminedangle.
 16. A method as claimed in claim 15, the method comprising:placing a material between a substrate and a mould, the mould having ashaped surface, at least a portion of the shaped surface having analignment layer formed thereon, and the substrate having a first surfaceon which is formed a bonding layer; bringing the mould and the substratetogether so as to sandwich the material between the first surface of thesubstrate and the shaped surface of the mould; polymerising the materialso as to form said first material; adhering the material to the bondinglayer; removing the substrate with the adhered polymerised material fromthe mould; covering the shaped surface of the polymerised first materialwith a polymerisable further material; and polymerising the furthermaterial so as to form the second material.